1. Field of the Invention
The present-invention relates to a luminescent panel for color video display used as picture elements of a color video display apparatus, and its driving system. The invention also relates to a color video display apparatus utilizing the same.
2. Description of the Related Art
In a color video display apparatus capable of displaying an image on a large screen, many luminescent panels are arranged in two dimensions to form the large screen. Each luminescent panel corresponds to one or more picture elements. One type of the luminescent panel utilizes a fluorescent lamp capable of efficiently providing sufficient area brightness, which is disclosed in Japanese Laid-Open Patent Publication No. 2-129847, and its corresponding European Patent Application EP-A-0,372,234. Referring to FIG. 1, the structure of such a luminescent panel 100 will be described.
The luminescent panel 100 has a cylindrical container 2 housing a coil filament 1 serving as a cathode, a casket 4 in which six discharge rooms 3a-3f are partitioned, and a light-transmitting front plate 5. These constitute a hermetic container. Generally, the coil filament 1 is a tungsten electrode on which an oxide layer is formed. The oxide layer serves as an emitter which emits thermoelectrons by a current flow. Anodes 6a-6f are respectively provided in the discharge rooms 3a-3f, and mixed gas of mercury vapor and rare gas is confined therein as discharge gas.
A phosphor layer for emitting light (not shown) is provided on an inner wall of the casket 4. More specifically, for example, the phosphor layer in the discharge rooms 3a and 3d is for green light, the phosphor layer in the discharge rooms 3b and 3e is for red light, and the phosphor layer in the discharge rooms 3c and 3f is for blue light.
The luminescent panel 100 is a hot cathode type, and its specific mechanism for emitting light will be described hereinafter.
When an electric current flows through the coil filament 1, thermoelectrons are emitted from the oxide emitter formed on the surface of the coil filament 1. The thermoelectrons ignite discharge in the discharge rooms. The discharge excites the mercury vapor in the mixed gas confined in the discharge rooms 3a-3f so that ultraviolet light is generated. When the ultraviolet light irradiates the phosphor layer on the inner wall of the casket 4, light of a predetermined color is emitted.
By arranging the luminescent panels 100 in a matrix, a color video display apparatus displaying television images or the like can be constructed. In this case, one picture element is constituted by three discharge rooms 3a-3c, and another picture element is constituted by the other three discharge rooms 3d--3f. Therefore, one luminescent panel 100 corresponds to two picture elements.
Although the luminescent panel 100 of the hot cathode type requires a high voltage of approximately 300 V to ignite the discharge, the discharge is sustained by applying only a voltage of approximately 40 V thereafter. In addition, its luminous brightness is substantially in proportion to the value of a current emitted from the coil filament 1.
Other than the hot cathode type, a cold cathode type is also used as the luminescent panel for a color video display apparatus. According to the cold cathode type, the discharge gas is ionized by applying a high voltage between metal electrodes, thus the discharge occurs. Since the filament is not used in the luminescent panel of the cold cathode type, unlike in the luminescent panel of the hot cathode type, its size can be easily miniaturized. Thus, an array pitch of picture elements can be narrowed.
In the luminescent panel of the cold cathode type, however, it is necessary to always apply a high voltage of approximately 200 V to the discharge tube or to the current confining element in order to sustain the discharge. Therefore, energy efficiency is lower in the cold cathode type compared to in the hot cathode type because the cold cathode type requires a higher voltage so as to sustain the discharge. Especially in a large-sized color video display apparatus having a large screen, since the required number of luminescent panels increases as the screen size increases, improvement of energy efficiency is an important factor. Thus, the luminescent panel of the hot cathode type is an indispensable component in the video display apparatus which does not need a picture element pitch of the order of submillimeter.
However, the conventional luminescent panel 100 of hot electron type and the conventional color video display apparatus utilizing such a panel have the following problems.
The number of picture elements obtained by one luminescent panel 100 is limited to two according to the aforementioned structure. In addition, since the coil filament of a certain length is necessary, it is difficult to narrow the array pitch of the picture elements to approximately 10 mm to 30 mm or less by reducing the size of the luminescent panel 100. Therefore, when the number of picture elements is increased to improve resolution of a displayed images, the required number of the luminescent panels 100 is increased and the display screen becomes huge beyond necessity. Thus, external wirings of the luminescent panel 100 becomes complicated. In addition, due to such a huge display screen, it is difficult to apply it to a color video display apparatus to be used indoors.
Additionally, since six discharge rooms 3a-3f are arranged on the right and left sides around one coil filament 1, an amount of thermoelectrons supplied to each of the discharge rooms 3a-3f is not likely to be uniform. As a result, a voltage for sustaining the discharge in each of the discharge rooms 3a-3f varies, and brightness is not likely to become uniform. In order to solve the above problems, it is thought to provide a plurality of coil filaments 1 in the cylindrical container 2 in one luminescent panel 100. In this case, however, the total amount of heat released during operation increases, so that the temperature is likely to be increased beyond the optimum operation temperature of the luminescent panel 100. Consequently, the luminous brightness is reduced, while the likelihood of damage to the luminescent panel 100 is increased.
In order to solve the above-mentioned problems regarding the conventional luminescent panel of the hot cathode type, it is necessary to provide a new hot cathode type luminescent panel.